Method of forming crystal

ABSTRACT

A crystal is formed by applying crystal forming treatment to a substrate, the surface of the substrate being divided into nonnucleation surface exhibiting a small nucleation density and nucleation surface having a sufficeintly small area to allow crystal growth from a single nucleus and exhibiting a larger nucleation density than the nonnucleation surface and the nonnucleation surface being constituted of the surface of a buffer layer to alleviate generation of stress in the crystal formed.

This application is a continuation of application Ser. No. 08/222,367filed Apr. 4, 1994, now abandoned, which is a continuation ofApplication Ser. No. 08/014,002 filed Feb. 5, 1993, now abandoned, whichis a continuation of application Ser. No. 07/689,104 filed Apr. 23,1991, now abandoned, which is a continuation of Application Ser. No.07/234,750 filed Aug. 22, 1988, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of forming a crystal,particularly a method of forming a crystal utilizing the difference innucleation density of a material to be deposited depending on the kindsof deposition surface materials.

The present invention is applied, for example, to formation of crystalsincluding monocrystal, polycrystal, etc, used for electronic devicesoptical devices, magnetic devices, piezoelectric devices, surfaceacoustic devices, etc. of semiconductor integrated circuits, opticalintegrated circuit, magnetic circuits, etc.

2. Related Background Art

There are described experimental results with regard to the selectivityof crystal formation on various substrate materials in Claassen et al,J. Electrochem. Soc., Vol. 127, No. 1, 194-202 (January 1980).

In the above paper, Claassen et al experimentally confirm that theselective silicon crystal formation on substrates can be performed, bypreparing two kinds of substrates, one having a surface constituted of amaterial facilitating the deposition film formation thereon (Si₃ N₄) andthe other having a surface constituted of a material preventing the easyformation of a deposition film thereon (SiO₂). Claassen et al maysuggest that the selectivity of crystal formation described in the abovepaper can be utilized for selective formation of a crystalline depositedfilm on any substrate. However, Claassen et al give no teachings as to amethod for forming a monocrystal at a desired position on the surface ofa given substrate. Also, Claassen et al give no teachings as toformation of a polycrystal having a precisely controlled crystal grainsize.

On the other hand, a method for forming a polycrystal by formingindividual monocrystal grains with a desirably controlled grain size atdesired positions on a substrate is described in European PatentApplication No. 0244081 in the name of the present assignee.

The above method is illustrated, for example, in FIGS. 1A-1D. Asubstrate having a support 101 and a nonnucleation surface 102 formedthereon constituted of a material preventing easy formation of a crystalnucleus with nucleation surfaces 103-1, 103-2 constituted of a materialfacilitating formation of a crystal nucleus and patterned to have aminor area (FIG. 1A) is subjected to a crystal growth treatment in vaporphase to allow monocrystals to grow on the nucleation surfaces 103-1,103-2, thereby performing selective growth of crystal islands 104-1,104-2 as individual monocrystal regions on the substrate (FIG. 1B). Whena plurality of crystal islands grow to be brought into contact with eachother and form grain boundaries 105, a polycrystalline film constitutedof an aggregation of monocrystal grains controlled in grain size andarrangement is formed. This method has the particular advantage thatsince nucleus generation and monocrystal growth can occur only on thenucleation surfaces 103-1, 103-2, monocrystals with a desired size areformed at desired positions.

In the course of further investigation of the above method, the presentinventor has found out the fact that crystal defects in the crystals mayoften occur at the proximity of the interface between crystals 104A-1,104A-2 and the nonnucleation surface 102 in the stage of crystal lateralgrowth on the nonnucleation surface 102 (FIG. 1B→FIG. 1C).

Although the crystal defects in some cases stop within the part at theproximity of the interface, they may be developed to the upper part ofcrystals to thereby cause a wide variation of characteristics among theindividual crystal islands. In case a large number of crystal islandsgrown on a substrate are utilized to form a large area active device,the above variation of characteristics among the individual crystalislands is a serious problem.

In other words, a crystal having a large number of crystal defects isinferior in electrical characteristics as compared to a crystal havingno defect, and in case a large area active device is formed by utilizinga large number of crystal islands, variation among devices is emphasizedby the variation of characteristics among crystal islands due to crystaldefects, thus lowering the reliability.

The present inventor, through intensive study, has developed a crystalforming method solving the above problem.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method of forming acrystal solving the currently recognized problem in the crystal formingtechnology as described above and suppressing the generation of crystaldefect.

Another object of the present invention is to provide a method offorming a crystal utilizing the selective nucleation technique whichutilizes the difference in nucleation density of a material to bedeposited on two kinds of deposition surface materials to formmonocrystal islands selectively on given positions on the depositionsurface wherein said deposition surface is the surface of a buffer layer(second layer) so that the generation of stress at the interface isgreatly suppressed as compared with a case employing substantially noconstitutional feature of the present invention.

Still another object of the present invention is to provide a method offorming a monocrystal at a desired position on any kind of support withsuppressing the generation of crystal defect.

Still another object of the present invention is to provide a method offorming a polycrystal controlled in grain size and the position of grainboundary with suppressing the generation of crystal defect.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1D are diagramatical views illustrating a crystal formingprocess utilizing selective nucleation. This process was proposed by thepresent applicant.

FIGS. 2A-2E are diagramatical views illustrating a silicon crystalforming process utilizing selective nucleation with employing a bufferlayer.

FIG. 3 is a graph showing the relation between the thickness of bufferlayer and the defect density of interface.

FIGS. 4A-4D are diagramatical views illustrating another crystal formingprocess utilizing selective nucleation with employing a buffer layer.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In a preferred embodiment of the present invention, the materialconstituting a nonnucleation surface is deposited to a thickness ofseveral hundreds angstrom units or less, i.e. as a buffer layer (hereina layer functioning to alleviate the stress of interface), so as toreduce stress generated at the interface between the monocrystal and thenonnucleation surface during crystal growth where a monocrystal formedon a nucleation surface grows laterally over the nucleation surface (onthe nonnucleation surface) and thereby suppress generation of defects inthe monocrystal at the proximity of the interface.

The above function is considered to be given when the layer constitutingthe nonnucleation surface has a thickness of several hundreds angstromunits or less. Because, it is considered that in such a case, the layeris in a state easily causing structural change and some change actuallyoccurs at the monocrystal growth temperature according to themonocrystal growth, thereby removing the cause for stress generated atthe interface.

The buffer layer is preferably formed at a temperature lower than thecrystal growth temperature, usually at a substrate temperature of70°-450° C., more preferably at 100°-350° C., optimally 100°-300° C. Thebuffer layer formed at such a low temperature, for example, at atemperature lower than the crystal growth temperature by 100° C. ormore, generates substantial thermal stress inside as heated to thecrystal growth temperature. When a crystal is allowed to grow on thebuffer layer under a state containing large thermal stress, the bufferlayer causes structural change in the course of crystal growth. As aresult, generation of stress at the interface between the monocrystaland the buffer layer is reduced and thereby generation of defect in themonocrystal at the proximity of the interface is suppressed.

If the buffer layer is thick (e.g. 1000 Å), structural change isprevented from extending smoothly over the whole film region and stressoften remains at the lower surface of the crystal. Since the remainingstress is a cause of defect generation, buffer layer should be usually500 Å or less, more preferably 200 Å or less, optimally 60 Å or less.The lower limit may be preferably 20 Å, more preferably 40 Å. Thethickness of the buffer layer is thus determined preferably within therange between the above upper and lower limits depending on the materialconstituting the buffer layer, the condition of forming the bufferlayer, the kind of crystal, the condition of forming the crystal, etc.In the present invention, the buffer layer thickness is preferably20-200 Å, more preferably 40-60 Å.

As a method for forming a buffer layer, an unequilibrating methodleaving thermal stress is desirably employed. For example, vapordeposition, the molecular beam epitaxial growth method, the lowtemperature MOCVD method, the plasma deposition method, etc. effectivelygenerate thermal stress in a buffer layer, thus being practicallypreferable. On the contrary, the high temperature CVD method, the LPEmethod (liquid phase growth method), etc. require appropriatemodification but is not unemployable in the present invention.

Further, the buffer layer must have a property functioning as anonnucleation surface as well as functioning for reduction of stressgeneration in a crystal. The buffer layer should preferably beconstituted of a material of which surface is lower in nucleationdensity by 10³ /cm² or more as compared to the nucleation surface. As apreferred material meeting the above requirement, SiO₂ by the sputteringmethod and silicon nitride by the plasma CVD method may be employed, butthe material is not limited to these materials, and those materialsfunctioning to reduce generation of stress can also be employed.

The nucleation surface has a sufficiently small area to allow crystalgrowth from a single nucleus. Specifically, the area is preferably 16μm² or less, more preferably 4 μm² or less, optimally 1 μm² or less.

In addition, the nucleation density of the nucleation surface must belarger than that of the nonnucleation surface, and the materialconstituting the nucleation surface should preferably be amorphous.

The present invention is described below specifically by referring toexamples.

EXAMPLE 1

FIGS. 2A-2E illustrate diagramatically the steps of the process forforming silicon crystals of low defect density by utilizing theselective nucleation according to the present invention.

(Step A)

On a high melting point glass support 201 was deposited a thin film 202of amorphous Si₃ N₄ to a thickness of 1500 Å by the LPCVD method as amaterial of high nucleation density enabling the selective nucleation(FIG. 2A).

(Step B)

On the Si₃ N₄ film formed was deposited an SiO₂ film 203 to a thicknessof about 50 Å by the sputtering method (FIG. 2B). The substratetemperature during deposition was maintained at 300° C.

(Step C)

A window 204 of 1 μm square was opened in the SiO₂ film 203 to expose apart of the Si₃ N₄ film, thus forming a seed for selective nucleation bythe conventional photolithography technique (FIG. 2C).

(Step D)

Next, HCl gas was allowed to flow on the surface of the substrate (i.e.support 201, Si₃ N₄ layer 202 and SiO₂ layer 203 with windows 204)maintained at a temperature as high as 970° C. to clean the substratesurface. Then, H₂, SiH₂ Cl₂ and HCl were supplied at a mole ratio of100:1.2:1.6 to thereby form a silicon monocrystal island 4 under thereaction pressure of 105 Torr and the substrate temperature of 920° C.(FIG. 2D). During this step, the SiO₂ thin film 203 as deposited at alow temperature functions continuously to alleviate stress generated inthe course of silicon crystal growth.

(Step E)

Crystal growth in Step D was continued for a desired duration to allowthe monocrystal island 4 to grow larger and eventually become apolycrystal 205 of a desired grain size (FIG. 2E).

Evaluation of the polycrystal was performed as follows. End face portionof the grown polycrystal 205 was polished and subjected to adefect-visualizing etching (so-called Secco etching). The subsequent SEMobservation indicated that the above-formed polycrystal had crystaldefects extremely lower in the density and magnitude thereof at theproximity of crystal end face as compared with a silicon polycrystalformed without interposing a thin buffer layer on an SiO₂ film which isformed to have a thickness of 1500 Å by the CVD method.

EXAMPLE 2

This example shows that the dependency of the number of defects at theproximity of crystal end face upon the thickness of buffer layer wasconfirmed experimentally.

FIG. 3 is a graph showing the relation between the thickness of a bufferlayer and the defect density at the proximity of crystal end face of anSi monocrystal island grown on the buffer layer. The defect density wasdetermined by polishing the end face portion with diamond paste, thensubjecting the polished surface to etching for 90 seconds with anetching liquid prepared by diluting to double the Secco etching liquidwith water to visualize the defects, and subsequently observing thedefects with the use of a SEM. Defects crossing a segment of 50 μmparallel to the polished surface by 1 μm were counted to obtain thedefect density.

Conditions of preparing the buffer layer and the Si crystal (Highmelting point glass supports were employed for each support for bufferlayer formation) are shown below.

(1) Conditions for Forming an SiO₂ buffer layer

RF sputtering method

Substrate temperature: 250° C.

Pressure: 6 mTorr

Use gas: Ar, O₂

Mole ratio: Ar:O₂ =2:3

RF power: 1.0 W/cm²

(2) Conditions for Forming an Si monocrystal

CVD method

Substrate temperature: 920° C.

Pressure: 150 Torr

Use gas: SiH₂ Cl₂, HCl, H₂

Mole ratio: SiH₂ Cl₂ :HCl:H₂ =0.6:1.0:100

As shown in FIG. 3, when the thickness of buffer layer decreases to 500Å or less, the number of defect decreases suddenly and this tendency isaccelerated with the decrease of buffer layer thickness.

EXAMPLE 3

This example illustrates a process for growing GaAs crystals of lowdefect density according to the crystal forming process utilizingselective nucleation of the present invention by referring to FIGS.4A-4D.

(Step A)

On a silicon wafer substrate 401 was deposited an amorphous siliconnitride film 402 to a thickness of 60 Å as a buffer layer by the plasmaCVD method using SiH₄ and NH₃ as starting material gases (FIG. 4A). Moleratio of SiH₄ to NH₃ supplied was 1:1, reaction pressure was 0.15 Torrand RF power (13,56 MHz) was 1.6×10⁻² W/cm².

(Step B)

After patterning with photoresist 403 (OSPR 800, supplied by Tokyo OkaK.K.), As ions were implanted to the silicon nitride layer 402 by usingan ion implanter (cs 3000 mfd. by VARIAN (FIG. 4B)). Inplanted amountwas 3×10¹⁵ /cm². Thus, amorphous nucleation surfaces 404 were formed.The ion implanted areas were 1 μm square and the interval thereof was 10μm. (Although FIGS. 4A-4D depict "one" nucleation surface 404, theactual number thereof was 10×10.)

(Step C)

Photoresist 403 was then removed, and the substrate (i.e. support 401and buffer layer 402 provided with nucleation surface 404) was thermallytreated for 10 minutes at about 900° C. under H₂ atmosphere to clean thesurface. Subsequently, while the substrate was heated at 650° C.,trimethyl gallium (TMG) and arsine (AsH₃) were allowed to flow on thesubstrate surface at a mole ratio of 1:60 together with H₂ as a carriergas, thereby growing a GaAs film by the MOCVD (metal organic chemicalvapor deposition) method. Reaction pressure employed was about 10 Torr.GaAs monocrystal islands grow only on the surfaces of As ion implantedportions 404, and no generation of GaAs monocrystal island was observedon the other surface portions (FIG. 4C).

(Step D)

After five hours growth, GaAs monocrystal island 405 became as large ascontacting with the adjacent GaAs monocrystals (FIG. 4D; 405A). The SEMobservation as in EXAMPLE 2 indicated no defect generation.

Though illustrated above in detail, the present invention is not limitedto the examples and includes various combinations of buffer layer (orsecond layer) condition, buffer layer forming condition and monocrystalforming condition.

As above, the present invention provides novel and effective means forsolving the problem involved in the monocrystal forming processutilizing the difference of nucleation density and makes a greatcontribution toward the monocrystal formation technology.

I claim:
 1. A method for suppressing the generation of defects in amonocrystal which defects tend to form in applying crystal formingtreatment to a substrate, comprising:(a) minimizing generation of stressduring said crystal forming treatment by providing a substrate having asurface which is divided into a nonnucleation surface subject to thermalstress at an interface with a growing monocrystal and a nucleationsurface having a sufficiently small area so as to form only a singlenucleus from which said monocrystal is grown and having a largernucleation density than the nucleation density of said nonnucleationsurface and said nonnucleation surface being a buffer layer sufficientto minimize the generation of thermal stress during a crystal formingstep and having a thickness of at most 500 Å, wherein the surface ofsaid buffer layer is constituted of a material having a nucleationdensity smaller by at least 10³ /cm² than the nucleation density of saidnucleation surface; (b) forming by vapor deposition a single nucleus onsaid nucleation surface; and (c) growing said monocrystal from saidsingle nucleus by vapor deposition whereby said buffer layer causesstructural change during the growing of said monocrystal and reducesgeneration of stress at the interface with said growing monocrystal. 2.A method of forming a crystal as defined in claim 1, wherein said bufferlayer is formed at a substrate temperature of 450° C. or lower.
 3. Amethod of forming a crystal as defined in claim 2, wherein said bufferlayer is constituted of an oxide or a nitride.
 4. A method of forming acrystal as defined in claim 3, wherein the thickness of said bufferlayer is 200 Å or less.
 5. A method of forming a crystal as defined inclaim 1, wherein said buffer layer is formed within a temperature rangeof lower than the substrate temperature during said crystal formingtreatment by 100° C. or more but not lower than 70° C.
 6. A method offorming a crystal as defined in claim 1, wherein said buffer layer isconstituted of either of an oxide and a nitride and has a thickness of200 Å or less.
 7. A method of forming a crystal as defined in claim 1,wherein said buffer layer is constituted of either of an oxide and anitride and causes structural change at the temperature of crystalforming treatment.
 8. A method of forming a crystal as defined in claim1, wherein said buffer layer is constituted of either of an oxide and anitirde and is formed within a temperature range of lower than thesubstrate temperature during said crystal forming treatment by 100° C.or more but not lower than 70° C.
 9. A method of forming a crystal asdefined in claim 1, wherein said buffer layer is formed at 450° C. orlower and causes structural change at the temperature of crystal formingtreatment.
 10. A method of forming a crystal as defined in claim 1,wherein said buffer layer is formed at 450° C. or lower and has athickness of 200 Å or less.
 11. A method for suppressing the generatingof defects in a monocrystal, which defects tend to form in applyingcrystal forming treatment to a body, comprising:(a) minimizinggeneration of stress along said crystal forming treatment by providing abody comprising a support; forming a first layer of an amorphousmaterial on the surface of said support; and forming a second layersubject to thermal stress at an interface with a growing monocrystal onthe surface of said first layer thereby exposing an area of said firstlayer sufficiently small so as to form only a single nucleus from whichsaid monocrystal is grown, said second layer being a buffer layer havinga surface with a smaller nucleation density than said first layer andhaving a layer thickness of at most 500 Å, wherein the surface of saidbuffer layer is constituted of a material having a nucleation densitysmaller by at least 10³ /cm² than the nucleation density of said firstlayer; (b) forming by vapor deposition said single nucleus in said firstlayer utilizing the difference of nucleation density between said firstand second layers; and (c) growing said monocrystal from said singlenucleus on the exposed area of said first layer and onto the surface ofsaid second layer by vapor deposition, whereby said buffer layer causesstructural change during the growing of said monocrystal and reducesgeneration of stress at the interface with said growing monocrystal.